Fibre lasers and fibre amplifiers are well known and are widely used. One form of fibre laser (or amplifier) comprises a double-clad active optical fibre. A double-clad active fibre typically comprises a core, an inner cladding which surrounds the core, and an outer cladding which surrounds the inner cladding. The core is doped with an active dopant material and guides and amplifies signal radiation which travels along the fibre. The inner cladding is configured to receive and guide pump radiation along the fibre. Using the inner cladding to receive and guide the pump radiation allows lower brightness pump radiation to be used to pump the fibre (compared with the case if the pump radiation were to solely be received by the core).
It is desirable to provide a laser or optical amplifier which is novel and inventive over the known prior art.